(1) Field of the Invention
This invention relates to a biaxially drawn polyhexamethylene adipamide film having improved toughness and appearance and to a process for producing the same. The biaxially drawn polyhexamethylene adipamide film of the invention is characterized as possessing improved toughness, good dimensional stability and a much reduced degree of yellowing.
(2) Description of the Prior Art
In the field of food packaging films, the degree of reliability required has been increasing, and accordingly, films having an enhanced toughness have been desired. Recently, in order to meet the requirements of enhancement of quality and the increase of varieties in retorted food, high-temperature short-time sterilization conditions, that is, highly retorting conditions, have been adopted instead of the conventional retort sterilization conditions, and therefore, in the field of retorted foods, the development of films having an excellent dimension stability sufficient to resist such high-temperature short-time sterilization conditions and an enhanced toughness capable of resisting rough and severe handling in such high-temperature short-time sterilization treatment has eagerly been desired.
In films for retortable pouches of foods, the excellent dimensional stability means that the films exhibit a much reduced change in dimension, i.e., a very low shrinkage factor, when the films are subjected to a retorting treatment with hot water or steam. If the change in dimension is large (i.e., the shrinkage factor is large), the food-packed pouch is deformed, resulting in a drastic reduction of the commercial value. Accordingly, high dimensional stability is indispensable for a film for a food-packaging retortable pouch.
Needless to say, the toughness of a packaging film should be such that the content in a pouch can be reliably protected and breakage or formation of holes, which will inevitably result in reduction of the commercial value, is not caused at all. Even if one pouch is broken among 10,000 pouches, the confidence of consumers to food-packed retortable pouches will be drastically lowered because in the present commercial distribution process for retorted foods and the like, the quality of the packed food or the like is degraded at a normal temperature in a short time if the pouch is broken.
Among various films now manufactured on an industrial scale, biaxially drawn nylon-6 films are used because of their high toughness and polyethylene terephthalate films (hereinafter referred to as "PET films" for brevity) are used because of their high dimentional stability. However, there has not been developed a film which is satisfies simultaneously both of these properties.
We previously succeeded in developing a biaxially drawn polyhexamethylene adipamide film capable of satisfying the foregoing two requirements simultaneously and proposed this film in the specification of U.S. patent application Ser. No. 956,267, now U.S. Pat. No. 4,252,966. Although this proposed polyhexamethylene adipamide film has simultaneously a good dimensional stability and a high toughness, this film fails to have the much enhanced toughness recently required in the art and is not attractive in appearance because it is considerably yellowish. The yellowish hue of a film leads to an undesirable appearance of a food-packed pouch and the commercial value thereof is drastically reduced.
We therefore did researches with a view to improving the biaxially drawn polyhexamethylene adipamide films proposed in U.S. patent application Ser. No. 956,267, now U.S. Pat. No. 4,252,966, and it was found that the above-mentioned three important properties are significantly influenced by the heat treatment step for imparting the dimensional stability in the process for the preparation of drawn films. More specifically, it was found that, when the conventional heat treatment, for example, the relax heat treatment disclosed in Japanese Patent Publication No. 534/62, the high-temperature short-time heat treatment disclosed in Japanese patent application No. 20,874/69 or the heat treatment taught in the above-mentioned U.S. patent application Ser. No. 956,267, now U.S. Pat. No. 4,252,966, is employed in the process of preparing drawn polyhexamethylene adipamide films, the toughness is lowered and the degree of yellowing becomes conspicuous, although there is an increase of the dimensional stability.
The relation between the dimensional stability and the toughness will now be described in detail with reference to FIG. 1. The value of the hot water shrinkage factor, which is an important property for a film for a food-packaging retortable pouch, is plotted as the criterion indicating the dimensional stability on the abscissa in FIG. 1, and the penetration strength is plotted as a typical criterion of the toughness on the ordinate in FIG. 1. In FIG. 1, the relationship between the hot water shrinkage and the penetration strength, observed in a biaxially drawn polyhexamethylene adipamide film heat-treated according to a conventional technique, is illustrated. Each value of the hot water shrinkage factor in FIG. 1 is a mean value of the values as measured, after dipping in hot water at 135.degree. C. for 10 minutes, on both the longitudinal direction and transverse direction of the film.
From FIG. 1, it will readily be understood that if the hot water shrinkage factor is reduced, that is, the dimensional stability is increased, the penetration strength, that is, the toughness, is reduced. The penetration strength is drastically reduced when the hot water shrinkage factor is 5% or lower.